After giving up a three-run homer to Jim Thome in the first inning, Millwood had to leave the game in the second with soreness in his right groin muscle -- an injury that had already forced him to leave a start this season and serve a stint on the 15-day disabled list.
"It got sore to where I couldn't push off and I couldn't throw any strikes when that happened," Millwood said. "I felt it at the beginning of the second inning."
Millwood had already gotten two outs and walked two batters in the second before pitching coach Mark Connor came out for a mound visit, followed by two team trainers.
After a short conversation on the field, Millwood left the game, being relieved by Josh Rupe.
"It's annoying; it's frustrating," Millwood said."I just want to get it taken care of and be able to pitch without worrying about it.
"It just seems like little things all over the place. I don't know what to do or what to say ... just try to get through it."
Rangers manager Ron Washington said Millwood will be evaluated Thursday to see if he can make his scheduled start Monday.
"We sent him out there today and we felt good about the way things were going," Washington said. "I wish I could explain how things happen in this game, but I can't. There was something that happened; we certainly didn't expect it. Since he came off the DL everything's been good. So we just have to wait until [Thursday] and see how everything goes."
Millwood is 6-6 with a 5.40 ERA in 20 starts this season. If he is unable to make his start Monday, Scott Feldman would be the likely replacement.
Feldman was brought up from Double-A Frisco to make a spot start last Monday.
David Just is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.